Field of the Invention
This invention relates to molding polyolefin containers and more particularly to molding polyolefin containers having improved gas barrier properties.
In the manufacture of canned foodstuffs, e.g., meat products as ham, corned beef hash, chili and beef stew, the containers, usually metal cans, are filled with the foodstuff, covered with a metal end closure and sealed.
One of the disadvantages of canning meat products in metal containers is that the presence of the food product may cause the interior of the container to corrode, which corrosion results in contamination and deterioration of the food product.
Attempts to substitute certain inert synthetic resin materials, such as polyethylene and polypropylene for metal in the canning of foodstuffs, have encountered the disadvantage that such resin materials are excessively permeable to gases, such as oxygen, and the permeation of oxygen into the container causes an undesirable discoloration and a depreciation in the taste and qualities of the foodstuff.
The high gas permeability characteristics of synthetic resins, such as polyethylene, has resulted in containers fabricated from such resins being rejected in the packaging of oxygen sensitive comestibles where due to the chemical inertness of the resin, it might otherwise be employed to great advantage.
The art has devised a number of ways to reduce the gas permeability of polyethylene and other polyolefin resins. Included in these methods is the fabrication of the container from a laminate formed from a plurality of layers of thermoplastic material, one of the layers being formed from a thermoplastic resin which exhibits high gas barrier properties, such as vinylidene chloride polymers, and acrylonitrile polymers, e.g. U.S. Pat. Nos. 3,453,173, 3,464,546 and 3,615,308. Other methods include incorporating in the thermoplastic resin a filler material such as wood flour, inorganic mineral fillers such as clay or mica, e.g., U.S. Pat. Nos. 3,463,350 and 3,668,038, or a second resin, such as nylon which has high gas barrier properties, e.g., U.S. Pat. Nos. 3,093,255 and 3,373,224. Although these prior art methods are effective in reducing the gas permeability of polyolefin resins, the gas permeability requirements for the most sensitive foodstuffs have still not been effectively met by containers fabricated from polyolefin resins.